The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, exhaust gas discharged from an exhaust manifold of an engine is induced to a catalytic converter that is mounted in the middle of an exhaust pipe and purified, and the noise thereof is reduced while passing through a muffler before the exhaust gas is discharged to the outside through a tail pipe.
The catalytic converter purifies the pollution materials that are included in the exhaust gas. Further, a particulate filter is mounted on the exhaust pipe to trap particulate material (PM) that is included in the exhaust gas.
A selective catalytic reduction (SCR) catalyst is a type of catalytic converter purifying NOx included in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide and hydrocarbon (HC) are provided to the exhaust gas, the NOx included in the exhaust gas is reduced by oxidation/reduction reaction with the reducing agents in the SCR catalyst.
Recently, a lean NOx trap (LNT) is used with the SCR catalyst to deal with reinforced exhaust gas regulations. The LNT absorbs NOx included in the exhaust gas if the engine operates in a lean atmosphere, and desorbs the absorbed NOx if the engine operates in a rich atmosphere, and thus reduces the desorbed NOx as well as NOx included in the exhaust gas.
In one form, when the LNT and the SCR catalyst are used together, the SCR catalyst may be coated on a diesel particulate filter due to space restriction. A selective catalytic reduction catalyst on diesel particulate filter (SDPF) absorbs particulate matter contained in the exhaust gas and eliminates NOx contained in the exhaust gas.
We have discovered that, in a predetermined driving condition, there is a difference between an NOx purifying efficiency detected by a sensor and a real NOx purifying efficiency. If a temperature gradient per unit time of the internal temperature of the SDPF is larger than a predetermined value, an NOx slip and an NH3 slip can be corrected by increasing or decreasing an amount of urea injection.
We have also discovered that if the temperature gradient per unit time of the internal temperature of the SDPF is not greater than the predetermined value, factors affecting the NOx purifying efficiency are changed according to the internal temperature of the SDPF. For example, when the internal temperature of the SDPF is in a low temperature region, absorption of the NH3 substantially affects a purifying performance, and space velocity (SV) of the SDPF, namely, volume flow rate per volume in the SDPF substantially affects an NOx purifying performance. By contrast, in a high temperature region, absorption rarely occurs and the NH3 directly reacts in a gas state, and thus the SV has relatively low impact on the purifying performance. In addition, since oxidation of NH3 (i.e., NH3→NOx transformation) occurs in a high temperature, the NOx purifying efficiency drops, therefore, output value of the NOx purifying efficiency map is adjusted in the high temperature.